Since the invention of xerography (which means "dry writing" in Greek) by C. Carlson in 1938, new facilities utilizing this technique such as xerox copier, laser printer and optical printer have provided inexpensive, convenient and fast services of copying documents and played important roles in office automation.
The focus of the xerographic technique resides in the electrophotoreceptor which is an optical element electrically insulative in darkness and becomes electrically conductive after exposure under light. The xerographic process comprises mainly five steps, namely, (1) charging, (2) photodischarging, (3) image transfer, (4) development and (5) cleaning. In order to obtain printed images of high quality, the photoreceptor must have high charge acceptance, low dark conductivity and fast photoconductivity (i.e., high sensitivity).
Photoreceptors can be classified as inorganic or organic. Typical inorganic charge generation materials include, for example, selenium, cadmium sulfide, zinc oxide and amorphous silicon. On the other hand, there are numerous organic charge generation materials, examples for which are photoconductive polymers such as poly-N-vinylcarbazole and polyvinylanthrancene, low molecular weight organic compounds such as carbazole, anthracene, oxadiazole, certain hydrazones and certain polyarylalkanes, organic pigments or dyes such as phthalocyanine pigment, azo pigment, cyanine pigment, polycyclic quinone pigment, perylene pigment, indigo dye, thioindigo dye and squaraine dye. Due to their advantages in low production cost, non-toxicity and high flexibility in utilization, organic photoreceptors (OPC) have replaced inorganic photoreceptors as the predominant photoreceptors among the commercialized photoreceptors.
The structures of photoreceptors may be classified as (1) mono layer type, such as that disclosed in U.S. Pat. No. 3,484,237, (2) functionally separated laminated type, such as those described in U.S. Pat. Nos. 3,837,851, 3,850,630, 4,123,270 and 4,293,628, and (3) microcrystalline distribution type. The functionally separated laminated layer type is the most preferred because it contains separated charge generation layer (CGL) and charge transport layer (CTL) and thus is highly flexible in the selection of materials for each layer. The characteristics and requirements may be adjusted independently in CGL or CTL. This type of photoreceptors are predominant among the present photoreceptors.
The functionally separated laminated type photoreceptors are generally composed of a conductive substrate, a charge generation layer and a charge transport layer. An optional barrier layer or an adhesive layer may be inserted between the conductive substrate and the charge generation layer. In the production of photoreceptors of this type, a charge generation layer composed of a charge generation material and a polymeric binder is coated on a conductive support and then a charge transport layer composed of a charge transport material and another polymeric binder is coated.
Organic charge transport materials have the advantages in multiplicity of selection and ease of synthesis. Extensive research therefore has been dedicated in this respect and organic charge transport materials have been becoming more important among present charge transport materials.
Organic photoreceptor may be produced by selecting suitable charge generation material, charge transport material and polymeric binders. A simple process with high productivity can be employed. However, conventional organic photoreceptors suffer from some disadvantages such as low sensitivity, high residual surface potential and bad reproducibility after repeated uses. The improvement of the these properties have always been sought after.